The invention relates to an electric machine with at least one electrical resistor for dissipating shaft voltages occurring at the rotor shaft of a rotor.
It is known that so-called shaft voltages can be formed on the rotor shaft during operation of electric machines, it being possible for said shaft voltages to have different causes. One cause for the occurrence of shaft voltages may be the clocked actuation of the stator winding or the rotor winding using a pulse-width-modulated signal. However, it is also possible that electrostatic charges lead to high shaft voltages. By way of example, an electric machine operated as a generator can be driven via a belt, with the result that the rotor can be electrically charged via the belt.
If the occurring shaft voltages discharge themselves in a pulsed fashion via a bearing in which the rotor shaft is mounted, this can lead to spark erosion in the region of the bearing surfaces and hence to damage of the bearing. In the case of electric machines which are operated in motor vehicles, higher shaft voltages occur the higher the supply voltage of the vehicle electrical system and hence the operating voltage of the electric machine is.
Furthermore, the pulsed discharge of the charge carriers located on the rotor shaft leads to the formation of electromagnetic waves, which negatively influence the electromagnetic compatibility of the product. In particular, this can prevent disturbance-free functioning of electrical devices located in the vicinity.
An arrangement for dissipating shaft voltages for electric machines is known from DE 35 11 755 A1, in the case of which arrangement charge carriers are dissipated from the rotor shaft via a sliding contact. For this purpose, a capacitance acting between rotor shaft and ground is provided, to which capacitance an inductance can be connected in series.
An alternating-current generator with means for suppressing the static electricity stored on the rotor of said generator is known from DE 101 18 004 A1, in the case of which generator the excitation winding of the rotor can be connected via an ohmic resistor to the shaft of the rotor.